Mold inserts

ABSTRACT

The invention relates to a mold insert, the interior surface thereof having an average quadratic roughness of less than 10 nm. Said insert is covered with a protective layer exhibiting a Vickers hardness of more than 1000 HV, a thickness of between 0.2 and 5 μm, and an adhesive strength greater than 15 N. The invention also relates to a method for the production thereof. The invention is used for the production of ophthalmic lenses.

[0001] The present invention relates to molding inserts and to theirmanufacturing process. It also relates to devices for molding opticalcomponents comprising such molding inserts, in particular for injectionmolding ophthalmic lenses made of polycarbonate.

[0002] The lenses are molded by injecting the material into a cavitywhich is formed especially by two inserts having the shape of the frontsurface and the rear surface of the lens to be molded, respectively.

[0003] These inserts are generally made of metal, but may also be madeof glass. The manufacturing process using injection molding subjectsthese inserts to very high mechanical and thermal stresses. However, theinserts must be able to undergo a large number of cycles in order toensure profitability of the process. Moreover, these inserts must beable to be easily cleaned, be chemically inert and be resistant toattack when being handled without becoming scratched. Production ofthese inserts is therefore tricky because of the number of constraintsto be taken into account.

[0004] With regard to the manufacture of aspherical unifocal lenses, itis well known to produce the inserts in the following manner: firstly,the metal block, generally made of steel, is machined to the desiredshape by means of a numerical control machine. Once machined, thesurface is polished. This surface is then coated with a chromium coatingso as to increase the abrasion resistance of the insert and thereforeits lifetime. The coating deposited has a thickness of between 40 and 60μm. Since the chromium coating is not very uniform, the external surfaceof the chromium coating no longer corresponds precisely to the shape ofthe machined surface. It is therefore necessary to polish the externalface of this coating so as to obtain the desired shape again. Althoughthis process is well-controlled, it nevertheless remains very expensive.

[0005] For inserts with a progressive surface, the process is even morecomplex as certain progressive surfaces require two-step machining: in afirst step, the surface is machined and checked by means of a specificapparatus for measuring progressive surfaces. A second step allows theobserved defects to be corrected. A chromium coating is then deposited,this then being polished, while still respecting the geometricalcharacteristics of the surface. This process, which includes arepolishing step, is very lengthy and expensive.

[0006] To control, or better still reduce, the cost of producing theselenses, it is important to reduce the cost of manufacturing the insertson the one hand and increase their lifetime in terms of the number ofcycles on the other hand.

[0007] Certain solutions aiming to simplify this insert manufacturingprocess have been proposed. For this purpose, it has been proposed todeposit carbon coatings with a uniform thickness so as to obviate therepolishing step.

[0008] Thus, patent U.S. Pat. No. 4,948,627 discloses the manufacture ofSiC molds for the hot-compression molding of precision optical elementsmade of glass. The mold is coated with a carbon coating intended tofacilitate demolding and comprising diamond and graphite crystallites aswell as amorphous carbon. The coating has a thickness of at most 1 μmand a maximum roughness of less than 20 μm. The coating is deposited bythermal or plasma-enhanced chemical vapor deposition (CVD). The gasphase comprises, apart from hydrogen, at least 3 mol % of an organicgas. However, the films containing graphite or hydrogen have a poortemperature resistance. This is because their hardness and theiradhesion to the mold decrease considerably after exposure to a hightemperature.

[0009] Patent U.S. Pat. No. 4,882,827 discloses SiC molds coated with amore temperature-resistant “hard” carbon coating. The “hard” carbon maycomprise diamond and graphite phases. The “hard” carbon coating isdeposited by sputtering in the presence of an inert gas such as argon ata temperature of between 250 and 450° C. The thickness of the layer isfrom 0.5 to 20 nm. The patent indicates that the coatings deposited byCVD lead to poor results. This type of process has the drawback ofrequiring the prior deposition of a β-SiC coating deposited by CVD so asto fill the pores of the mold and improve the surface finish. Thiscoating has a thickness of greater than 5 μm, typically 500 μm. Thecomplexity of the process, a coating of such a thickness no longer makesit possible to comply with the geometrical characteristics of thesurface of the mold for precision optical components.

[0010] Moreover, it turns out that such carbon coatings adhere poorly tometal inserts. To improve the adhesion, it has therefore been proposedto deposit an intermediate coating.

[0011] Thus, patent U.S. Pat. No. 5,872,613 discloses the production ofa DLC (diamond-like carbon) coating on a metal mold. The DLC ischaracterized by a Raman spectroscopy absorption peak at 1550 cm⁻¹. TheDLC coating has a thickness of between 0.1 and 100 μm, preferablybetween 1 and 50 μm. To improve the adhesion of the DLC coating to themetal mold, a molybdenum coating is interposed between the mold and theDLC coating. The deposition is carried out by ion evaporation at atemperature below 600° C. in the presence of a bombardment gas. Thistype of process requires the deposition of two coatings of differenttype, which slows down the manufacture. In addition, the processinvolves two different deposition techniques, which makes it expensiveand tedious.

[0012] Patent U.S. Pat. No. 5,112,025 discloses the coating ofnickel-plated molds for the manufacture of compact discs. The surface ofthese compact discs is plane and has patterns. The insert used toproduce such discs is in general covered with nickel, facilitating thetracing of patterns. Repeated injection molding of compact discs wearaway and flatten the patterns, resulting in a loss of geometriccharacteristics. Moreover, the injected material may remain blocked inthe patterns and cause difficulties in demolding. The document thereforeproposes to deposit a DLC protective coating on the nickel so as tofacilitate demolding. However, there is a risk that the durability ofthis type of “hard” coating deposited on a soft substrate, such asnickel, may not be suitable for the process of molding opticalcomponents, the inserts being exposed to considerable assault duringhandling operations, which is not the case with compact disc moldingdevices.

[0013] The problem that arises is then that of providing a device formolding optical components that does not have the drawbacks mentionedand especially has an improved lifetime and does not requirerepolishing.

[0014] According to the invention, this problem is solved by means of amolding insert whose internal surface has a mean square roughness Rs ofless than 10 nm, which insert is coated with a protective coating havinga Vickers hardness of greater than 1000 HV, a thickness of between 0.2and 5 μm, the critical adhesion force of which is greater than 15 N.

[0015] Preferably, the protective coating is made of DLC.

[0016] According to one embodiment, the insert bearing the protectivecoating is made of steel or glass.

[0017] In particular, the insert may bear markings.

[0018] Preferably, the thickness of the protective coating is between 2and 5 μm.

[0019] Advantageously, the insert coated with the protective coating hasan average surface roughness of less than 7 nm. According to a preferredembodiment, the protective coating has a Vickers hardness of greaterthan 1500 HV.

[0020] The subject of the invention is also a device for molding anoptical component that includes such an insert.

[0021] The subject of the invention is also the use of such a device formanufacturing ophthalmic lenses by injection molding. According to oneembodiment, the ophthalmic lens is a progressive lens. According toanother embodiment, the molded ophthalmic lens is made of polycarbonate.

[0022] Finally, the subject of the invention is a process formanufacturing a molding insert, comprising the steps of

[0023] machining and polishing of an insert until an R_(ms)of less than10 nm is obtained;

[0024] deposition of a protective coating having a Vickers hardness ofgreater than 1000 HV, a thickness of between 0.2 and 5 μm and thecritical adhesion force of which is greater than 15 N, preferably byCVD.

[0025] The Applicant has in fact found that it is possible to deposit aprotective coating, for example made of DLC, directly on the surface ofcomplex surfaces with sufficient adhesion to withstand the injectionmolding pressures if the substrate has a normalized roughness parameterR_(ms) (mean square roughness) of less than 10, preferably 7 nm and athickness of between 0.2 and 5 μm. An upper thickness of 2 μm of the DLCcoating also contributes to improving its abrasion resistanceproperties. It is then possible to obtain such a protective coating witha very uniform thickness, thereby making it possible to maintain thegeometrical characteristics of the surface of the mold and eveneliminating the tricky repolishing step.

[0026] The surface thus coated of an insert is more resistant to attack,and this makes it possible for its lifetime to be considerablyincreased. The process for manufacturing such devices is less expensivesince it requires neither deposition of an intermediate coating norrepolishing of the protective coating.

[0027] Furthermore, it has been observed that, unlike the insertsobtained by electrodeposition of chromium, the markings that certaininserts bear remain visible after the protective coating has beendeposited. This fact makes it easier to use these inserts havingmarkings, especially those intended for the manufacture of progressiveophthalmic lenses.

[0028] The invention will now be described more specifically below bymeans of a preferred embodiment.

[0029] A steel insert is machined and then polished. The surface finishpreferably has an R_(ms) of less than 10 nm and particularly less than 7nm.

[0030] A protective coating is deposited directly on this surface.Preferably, this is a diamond-like carbon coating. The diamond-likecarbon may be an a-DLC (amorphous diamond-like carbon), but also anMe-DLC (metal diamond-like carbon), such as WC-C, that is to say DLC inwhich metal components such as tungsten have been added). It may also beenvisioned to deposit TiN or TiCN.

[0031] The deposition is advantageously carried out by PACVD(plasma-assisted chemical vapor deposition). However, deposition by CVDor PVD is also possible.

[0032] The surface finish of the substrate, and therefore of the insert,is particularly important for obtaining good adhesion of the protectivecoating and guarantees its optical quality. Preferably, the surface ofthe insert has an average roughness of less than 10 nm, particularlyless than 7 nm.

[0033] Thickness uniformity ensures that the geometrical characteristicsof the insert are met without requiring repolishing. Preferably, thecoating has a thickness of between 0.2 and 5 μm. When the thickness isless than 0.2 μm, it no longer provides sufficient abrasion resistance.When the thickness is greater than 2 μm, improved adhesion is observed.When the thickness of the coating is greater than 5 μm, there is a riskof the geometrical characteristics of the surface and the markings nolonger being met. The coating thus deposited has a thickness uniformitysuch that it does not need to be polished, which makes it possible toeliminate an expensive and deforming step. Furthermore, since thethickness deposited is small enough, the geometrical characteristics ofthe surface are indeed met. Measurements performed by means of athree-dimensional measuring machine have shown that the thicknessvariation of the coating thus deposited is of the order of 0.1 μm, whichis very small and suitable for manufacturing ophthalmic lenses.

[0034] This manufacturing process makes it possible to obtainhigh-quality inserts, which in turn determines the quality of the lensesobtained by injection molding. This process, no longer requiring apolishing step, makes it possible to lower the production cost of theinserts. Finally, by increasing the lifetime of these inserts, such aprocess also makes it possible to lower the production cost of theophthalmic lenses obtained.

[0035] The coatings thus deposited have the following physico-chemicalcharacteristics. The Vickers hardness of the protective coating isgreater than 1000 HV and preferably greater than 1200 HV.

[0036] Preferably, the materials used ensure that there is a lowcoefficient of friction with the molding materials. Thus, a DLCprotective coating has proven to be compatible with the injectionmolding of polycarbonate. The protective coating is also preferablychemically inert.

[0037] The surface finish after deposition is also preferablycharacterized by a low roughness, for example characterized by a meansquare surface roughness R_(ms)<10 nm. In general, isolated defects ofthe crater or scratch type are not observed. Under these conditions, thecoatings exhibit good adhesion behavior. Typically, the critical forceof adhesion of the protective coating is greater than 15 N. The criticalforce L_(c) is the force expressed in newtons on the basis of which, ina scratch-type test with an imposed increasing normal force, there is noadhesion of the coating along the scratch. The test consists in moving aspecimen beneath a diamond tip (200 micron radius Rockwell indentor, MSTapparatus from CSEM). When the force applied to the diamond tip isincreased, the latter sinks progressively into the coating and producesa scratch of increasing depth. The value of the force at whichseparation of the coating from its substrate is observed is called thecritical adhesion force.

[0038] The value chosen for the thickness of the protective coatingsolves the problem that the markings present on the inserts pose. Thisis because progressive ophthalmic lenses necessarily bear markings whichare produced in negative form on the inserts, for example by laseretching. Each marking has a depth of about 0.9 μm and a width of about100 μm. These markings may therefore be deposited on the insert, beforedeposition of the protective coating, with the assurance that they willbe preserved.

1. A molding insert, the internal surface of which has a mean squareroughness R_(ms) of less than 10 nm, which insert is coated with aprotective coating having a Vickers hardness of greater than 1000 HV, athickness of between 0.2 and 5 μm and the critical adhesion force ofwhich is greater than 15 N.
 2. The insert as claimed in claim 1, inwhich the protective coating is made of DLC.
 3. The insert as claimed inclaim 1 or 2, in which the insert bearing the protective coating is madeof steel or glass.
 4. The insert as claimed in one of the precedingclaims, in which the insert bears markings.
 5. The insert as claimed inone of the preceding claims, in which the thickness of the protectivecoating is between 2 and 5 μm.
 6. The insert as claimed in one of thepreceding claims, in which the insert coated with the protective coatinghas an average surface roughness of less than 7 nm.
 7. The insert asclaimed in one of the preceding claims, in which the protective coatinghas a Vickers hardness of greater than 1500 HV.
 8. A device for moldingan optical component comprising an insert as claimed in one of thepreceding claims.
 9. The use of the device as claimed in claim 8 for themanufacture of ophthalmic lenses by injection molding.
 10. The use asclaimed in claim 9, in which the ophthalmic lens is a progressive lens.11. The use as claimed in claim 9 or 10, in which the molded ophthalmiclens is made of polycarbonate.
 12. A process for manufacturing a moldinginsert, comprising the steps of: machining and polishing of an insertuntil an R_(ms) of less than 10 nm is obtained; and deposition of aprotective coating having a Vickers hardness of greater than 1000 HV, athickness of between 0.2 and 5 μm and the critical adhesion force ofwhich is greater than 15 N.
 13. The process as claimed in claim 12, inwhich the protective coating is deposited by CVD.